Malignant tumors are often treated by surgical resection of the tumor to remove as much of the tumor as possible. Infiltration of the tumor cells into normal tissue surrounding the tumor, however, can limit the therapeutic value of surgical resection because the infiltration can be difficult or impossible to treat surgically. Radiation therapy can be used to supplement surgical resection by targeting the residual tumor margin after resection, with the goal of reducing its size or stabilizing it. Radiation therapy can be administered through one of several methods, or a combination of methods, including external-beam radiation, stereotactic radiosurgery, and permanent or temporary brachytherapy. The term “brachytherapy,” as used herein, refers to radiation therapy delivered by a spatially confined radioactive material inserted into the body at or near a tumor or other proliferative tissue disease site. Owing to the proximity of the radiation source, brachytherapy offers the advantage of delivering a more localized dose to the target tissue region.
Brachytherapy can be performed by implanting radiation sources directly into the tissue to be treated. Interstitial brachytherapy is traditionally carried out using radioactive seeds such as 125I seeds. These seeds, however, produce inhomogeneous dose distributions. In order to achieve a minimum prescribed dosage throughout a target region of tissue, a large number of seeds must be used, resulting in very high doses being delivered to tissue in proximity to the seed, which can cause radionecrosis in healthy tissue.
An alternative method of performing post-surgical brachytherapy is the implantation or insertion of an inflatable balloon catheter system into the resection cavity. For example, U.S. Pat. No. 5,913,813 (which is hereby incorporated by reference in its entirety) discloses a double-wall balloon catheter including a balloon, or distensible reservoir, that can pushing tissues away from close proximity to a radiation source. This allows the irradiated tissue to be in a region of shallow dose gradient and results in a reduced risk of “hot-spots” and radionecrosis.
In general, the amount of radiation desired by the physician is a certain minimum amount that is delivered to a region up to about two centimeters away from the margin of the excised tumor. It is desirable to keep the radiation that is delivered to the tissue in the target treatment region within a narrow absorbed dose range to prevent over-exposure to tissue at or near the radiation source, while still delivering the minimum prescribed dose at the maximum prescribed distance from the radiation source. It is also desirable, at least in some applications, to provide these advantages while tailoring the radiation dosage to avoid fully dosing sensitive tissue or to reduce the amount of radiation that escapes the patient's body.
Thus, there is a need for an instrument which can be used to deliver radiation from a radioactive source to target tissue within the human body with a desired intensity and at a predetermined distance from the radiation source without over-exposure of sensitive body tissues, such as skin or organ tissue for example, disposed proximate to the radiation.